1. Field of the Invention
This invention relates to free abrasive slurries suited for use in polishing workpieces composed of materials varying in hardness, uniformly without selective polishing or working without difference in the amounts of polishing for different materials. The invention also relates to a method of manufacturing thin film magnetic heads which involves height polishing with a free abrasive slurry composition suited for use in lapping of thin film magnetic heads provided with a slider having an air bearing surface (hereinafter called xe2x80x9cABSxe2x80x9d), uniformly without selective polishing or working without difference in the amounts of polishing for different materials.
2. Prior Art
In recent years there have been growing demands for optical, electronic, and precision equipment parts with more and more enhanced functions and performances than before. For those parts a very wide range of materials have come into use, including various metal crystal materials, ceramics, glass, and plastics. Consequently, polishing has found a broadening range of applications in the fields of parts composed of a plurality of materials varying in hardness. Typical of the polishing of such composite materials are the uniform finishing of conductive metal and interlayer insulation film in the course of multilayer interconnection of LSIs and in the end polishing of optical fiber connectors among optical components.
Meanwhile, improvements in hard disk drives have been made year after year for higher recording density of the disk, a recording medium for computers. As a means of achieving the higher recording density, efforts have been made for xe2x80x9clower head floatingxe2x80x9d, so called because the floating gap between each hard disk and the magnetic head is decreased and the disc-head space is narrowed down.
The magnetic heads mounted in hard disk drives are predominantly of the thin film type. They are built of a composite material, e.g., a ceramic such as AlTiC (Al2O3xe2x80x94TiC) as a base and metallic films of magnetic materials such as Permalloy (Fexe2x80x94Ni) and Sendust (Fexe2x80x94Alxe2x80x94Si).
Today the floating type magnetic heads are generally manufactured through a sequence of the following steps.
1. Bar cutting
The bar, as shown in FIGS. 1a-1b, is cut off from a wafer composed of a number of magneto resistive elements arranged in a matrix form. A plurality of sliders are arranged in a row. Shows a schematically a bar cut out from a wafer to form a magnetic head element as a workpiece to be polished.
2. Bonding the bar to a work jig (see FIG. 3).
3. Bar lapping (see FIG. 4).
By lapping is meant a process in which, as FIG. 4 illustrates, the ABS of a slider is polished by placing the workpiece on a plate made chiefly of tin or the like and being rotated with a constant supply of a free abrasive slurry or the like.
4. Separation of the bar from the jig.
5. Rail etching.
6. Cutting sliders away from the bar.
In the above sequence of steps, 3. Bar lapping is the step that involves the polishing concerned with the present invention. Among slider working methods the most common particularly for the air bearing surface consists in throat height polishing or MR height polishing (both being collectively called xe2x80x9cheight polishingxe2x80x9d) with a free abrasive slurry and final polishing as the final stage of the height polishing or after the height polishing.
The term xe2x80x9cthroat heightxe2x80x9d (Throat height: TH) as used herein means one of the factors that determine the record writing characteristics of a thin film magnetic head. As indicated at TH in FIG. 2, the throat height represents the length of a magnetic pole region or the distance from the ABS to the edge of an insulation layer that electrically isolates a thin film coil. Machining for finishing the throat height to a desired length is known as throat height polishing. Among thin film magnetic heads, those equipped with a magnetoresistance regenerative element are called MR heads, and a deciding factor for the recording-reproducing characteristics of an MR head is the height of its magnetoresistance regenerative element, known as MR height (or MR-h). The MR-h, as indicated in FIG. 2, represents the length as measured from the ABS of the magnetoresistance regenerative element whose end is exposed to the ABS. Machining to attain an MR-h of a desired length is known as MR-h polishing.
When the height polishing of the ABS of thin film magnetic heads made of ceramic-metal film composite material is carried out using a conventional free abrasive slurry, step-like irregularities result in most cases from the differences in hardness among the component materials, for example, by the selective polishing of the metallic films of Permalloy, Sendust, etc. which are soft materials used for the pole regions. The selective polishing of the pole material composed of the metallic films of Permalloy, Sendust, etc. leads to the recession of the metallic films in the pole and nearby regions from the ABS of ceramics.
The recession, known as pole tip recession or PTR, increases the magnetic distance from the recording medium, resulting in a substantial increase in the degree of floating of the head. For the prior art polishing with an ordinary free abrasive slurry, therefore, final polishing at the final stage of, or after, the height polishing has been essential to reduce the amount of recession from the floating surface that has resulted from the selective polishing of the pole region.
The final polishing is imperative also to moderate or eradicate the scratches and irregularities on the surface, especially of metallic films of Permalloy, Sendust, etc., caused by polishing with a conventional free abrasive slurry. The methods in common use for final polishing includes driving a turntable at a low speed, adjusting the load on the ABS during polishing, or discontinuing the supply of a free abrasive slurry and supplying instead an abrasive-free liquid, e.g., only the dispersion medium for the above-mentioned free abrasive slurry.
Another common practice for the final polishing is the use of a polishing equipment designed exclusively for that purpose.
In some cases the final polishing of the ABS is followed by a polishing (hereinafter called xe2x80x9ctaper polishingxe2x80x9d) to provide a taper in the portion on the inflow side of. the ABS. The taper polishing, intended solely for the polishing of ceramics, is usually performed by altering the polishing conditions and feed liquid once changed for the final polishing back to the original conditions for height polishing, or by the use of an exclusive polisher for the taper polishing. Thus the prior art methods of polishing thin film magnetic heads which involve one or more final polishing steps in which the polishing conditions and the slurry or other polishing liquid have to be changed stepwise, had productivity problems such as long polishing periods and the need of separate polishing apparatus for a plurality of steps.
As for the lubricants for the polishing of composite materials made of materials varying in hardness, oily agents, antiwear agents, or extreme pressure agents have been used.
Typical oily agents include fatty acids, aliphatic alcohols, aliphatic amines, aliphatic esters, and oils and fats. They are believed to be amphiphatic substances having a long hydrocarbon chain in the molecule terminated with a strong polar group and capable of forming a lubricating film by physisorption or chemisorption on surfaces that rub each other, so that the film keeps the surfaces out of direct contact and reduces their friction. Oily agents are usually applied under relatively moderate light to medium load conditions. Severe, high-temperature heavy-load conditions would break the film and deteriorate its lubricity.
Antiwear agents such as phosphoric esters and metallic dithiophosphates are believed to decrease wear by forming a lubricating film through a tribologic chemical reaction with friction surfaces under the conditions of light to medium loads and elevated temperatures.
Under heavy load conditions, however, the film is broken by strong shear and heat of friction generated.
Extreme pressure agents are able to form films of greater shear strength than the films of oily agents and antiwear agents under demanding conditions of high temperatures and heavy loads. These extreme pressure agents are considered to have two mechanisms of action.
One mechanism forms a film by reaction with a metallic surface, and the other forms a film without reaction with a metallic surface but as a result of thermal decomposition of the molecules of the extreme pressure additive on a frictional surface. When a composite material made of materials varying in hardness is polished, the differences in hardness among the component materials result in varied amounts of elastic deformation, with less hard materials being preferentially polished more than the remainder. In uniform polishing is to be achieved, a lubricating film that effectively eliminates the variation in the amount of elastic deformation becomes necessary. Further problems arise from the polishing using conventional free abrasive slurry compositions. Since the polishing takes the advantage of microscopic scratching action of the abrasive powder, the operation has to be carried out under the conditions of localized high temperatures and high shear rates. If, in order to overcome the difficulty, an oily agent or antiwear agent that exhibits lubricity under relatively mild conditions is employed as a lubricant, the lubricating film will sometimes be broken, causing scratches or surface roughness of metallic films such as of Permalloy and Sendust.
Sulfur-containing organomolybdenum compounds have been objects of investigations as antiwear agents (P. C. H. Mitchell: xe2x80x9cOil Soluble Mo-S Compounds as Lubricant Additivexe2x80x9d, Wear, 100 (1984), pp.281-300; xe2x80x9cSEKIYU SEIHIN TENKAZAI NO KAIHATSU TO SAISHIN GIJUTSUxe2x80x9d (Developments and Latest Technologies of Oil Product Additives), compiled by Heihachiro Okabe, CMC (1998), pp.99-106; etc.). They have not been studied, however, as lubricants for polishing such composite materials made of materials varying in hardness as are handled under the present invention.
The present invention aims to provide a polishing slurry that can solve the problem of selective polishing of the soft material in a workpiece that is composed of soft and hard materials and to provide a polishing method using that polishing slurry. More particularly, the invention aims to provide a polishing method which uniformly finishes the ABS of thin film magnetic heads that are made of a plurality of materials varying in hardness (that is, finishes without difference in the amounts of polishing among the different-hardness materials, or without selective polishing). The method enhances productivity because it permits polishing in a. single step without the need of changing the type of free abrasive slurry or replacing the polisher, at no sacrifice of the polished quality of ABS that is obtained by the conventional height polishing process.
The invention provides free abrasive slurry compositions for polishing composite materials made of materials varying in hardness.
The compositions include those comprising a polyol with a molecular weight of 300 to 20000, a polishing powder, a dispersion medium, and optionally a surfactant; and those comprising a sulfur-containing organomolybdenum compound, a polishing powder, a dispersion medium, and at least one member selected from the group consisting of surfactants, polymeric compounds, and surface modifiers.
The use of such a free abrasive slurry composition in the polishing of a material composed of constituents varying in hardness decreases the percentage of the regions where there is solid-to-solid contact and selectively lowers the friction coefficient of a less hard work surface. That means reducing the amount of removal of a component material which would otherwise be easily removed, resulting in uniform polishing without a difference in the amount of polishing between the materials varying in hardness.